Lighting Design Glossary

Photometry

Photometry is the science of measuring visible light in
units that are weighted according to the sensitivity of
the human eye.

The sensitivity of the human eye to light varies with
wavelength. A light source with a a given radiance of
green light, for example, appears much brighter than
the same source with the same radiance of red or blue light.
To convert radiometric and photometric units, the eye's
response is approximated by the CIE weighting function.

Photometric theory does not address how we perceive colors.
The light being measured can be monochromatic or a
combination or continuum of wavelengths.